This invention relates generally to the field of integrated circuits and more particularly to integrated circuits having a current limited charge pump.
Integrated circuits often employ circuitry such as charge pumps to increase or decrease voltage levels within the integrated circuit. Conventional charge pump designs typically include diodes to couple multiple stages. Such charge pumps typically utilize several stages to achieve a voltage step-up. Current losses within each of these stages may reduce efficiency and may result in poor predictability of the current flow out of the pump, thus complicating designs for current limiting. Adding stages may decrease the efficiency of such a pump due to forward drops in voltage across each diode.
Existing charge pumps sometimes employ current limiting circuitry at the output. One prior approach includes a current mirror coupled to the output of the charge pump. This approach harms power efficiency due to the loss of the current drawn down the reference limb of the current mirror. The size of the devices within the charge pump must also be increased to handle the additional current drawn by the current mirror. Since many charge pumps used in integrated circuits produce very small currents (1 uA or less), the increase in device size may be considerable.
One aspect of the invention is an integrated circuit comprising a current source coupled to an input element and an output load. The integrated circuit further comprises a first stage charge pump coupled to the current source at a first node and to the output load at a second node and a recirculation circuit coupled to the first node and the second node. The recirculation circuit is operable to limit to a known value the current that flows between the second node and the output load.
The invention provides several important technical advantages. The invention improves current efficiency while providing a current-limited output. The invention may utilize any charge pump. It achieves these advantages with simple circuitry that consumes little integrated circuit area.